A semiconductor stack including a structure in which an active layer containing antimony (Sb) as a group V element is arranged on a substrate composed of a III-V group compound semiconductor can be used for the production of a semiconductor device such as a light-receiving device sensitive to mid-infrared light. Specifically, for example, an infrared light-receiving device can be produced by using the active layer as a light-receiving layer, forming a contact layer on the light-receiving layer, and forming appropriate electrodes. To achieve a good crystallinity of the active layer, which has a potent influence on the performance of the light-receiving device, a buffer layer serving as an underlying layer of the light-receiving layer needs to have a good surface flatness.
A method for forming a semiconductor layer having good surface properties on a substrate that has a main surface inclined with respect to a crystal plane with a predetermined plane direction, i.e., an off-axis substrate, is reported (for example, see PTL 1).